Abstract:Studies on the origin and the whole developmental stages of juice sac of Citrus reticulata var. unshiu (Marcoritch) Hu indicate that the juice sac originates from a single cell of the carpellary inner epidermis. After anticlinal and periclinal divisions of the initial cell, the juice sac primodium forms a multicellular globular structure. With the development of citrus fruit, the globular structure developes into a columnar structure, in which tissue differentiation takes place simultaneously. At the center of the column apex, a meristem-like cell cluster differentiates. The meristem-like cells are polygonal, each cell has a big nucleus in the center surrounded by dense cytoplasm. The other part of the column consists of parenchyme cells. The column then expands and widens as a result of the division and growth of meristem-like cells. After three months of the fruit development, the columnar structure has grown into an expanding sac, espcially by the expansion of parenchyma cells filled with juice in the vacuoles. The mature juice sac is a swollen sac with a slender stalk and a sac wall lined with layers of tightly arranged cells. The mature juice sac stores an abundant amount of water, sugar, organic acid. pigments, etc. in the parenchyma.